The primary objective of this project is to identify possible markers for language impairment early in development. Over the last three decades, research on early language has revealed that infants possess vast knowledge of their language prior to producing their first word. Months before speaking, infants have amassed and pruned their linguistic systems in a way that prepares them to copiously produce speech as toddlers. This discovery expands the conventional trajectory of language development, which has traditionally only provided linguistic milestones after the child's first birthday. It is now theoretically possible to identify linguistic milestones in infancy and, by extension, to potentially identify infants who do not develop on course. This would serve to link the heretofore discrete fields of infant speech perception and child language acquisition, thereby confirming that the former truly reveals precursors to language production in childhood. Furthermore, it would invite the potential for early intervention and treatment of language disorders. Given that successful treatment for language disorders is highly time-sensitive, identification of a disordered trajectory in infancy would increase the probability of successful remediation. This application aims to characterize when and how infants grow a mental lexicon and how their progress in these areas bears on later word learning. Existing research on spoken word recognition has shown that infants represent words in memory by 7.5 months, although at this age, memories for words are relatively fragile, indiscriminately encoding episodic details (e.g., talker gender, vocal emotion). Later, by 10.5 months, infants have refined their representations to reflect only phonemic detail. Studies investigating how infants refine their word representations have discovered that the ability to categorize words according to their shared, invariant properties allows them to distill phonemic information from speech. This capacity is likely to be a function of phonological working memory, which in turn has been shown to influence vocabulary development and may predict later language development. The role of phonological working memory, as a possible contributor to word knowledge in infancy and childhood, is systematically explored. In sum, this application endeavors to determine whether infants' capacities to build and prune memories for words predict the facility with which they learn words and accumulate a vocabulary as children.